It is common to employ a relief valve on a pressurized cylinder so as to prevent premature rupture should the internal pressure become excessive. With the use of new lightweight material to form pressurized cylinders, the industry has experienced a new problem. Lightweight materials such as aluminum, Kevlar and plastic tend to lose strength at elevated temperatures and, if stressed, stretch and deform so that the wall thickness eventually becomes thinner. In such cases, the safety value of the cylinder is reduced and a catastrophic rupture could prematurely occur.
It can also be appreciated that a relief valve which opens at a certain pressure value gives some protection against overheating of the cylinder itself. However, under certain conditions, such as a partially filled cylinder or a cylinder made from a material which undergoes a relatively rapid decay in its tensile strength or other physical properties with increasing temperature, a pressure responsive valve may not be sufficient to relieve the pressure before the stresses induced in the cylinder exceed the recommended value. Because of this, it is advantageous to employ a relief valve which has both a temperature and pressure sensing mechanism which are independent of each other.
There are a number of patents directed to relief valves. These include U.S. Pat. Nos. 4,407,432; 4,352,365; 4,059,125; 3,472,427; and 2,040,776. Of these patents, the U.S. Pat. No. 4,352,365, issued to Boccardo et al, addresses the issue of providing both a temperature and pressure activated relief valve. However, Boccardo discloses a valve which uses a common outlet for releasing the pressurized fluid, while the present invention provides for two separate passageways which can allow for quicker venting.
Now a combination thermal or pressure activated relief valve has been invented which can release a fluid or a gas from a pressurized cylinder should its internal pressures exceed a predetermined value or should the cylinder itself become exposed to excessively high temperatures.